05 



The hornblende possesses the deep brown colour and strongly-marked 

 pleochroism of the variety known as "basaltic" hornblende. , pale 

 yellowish brown ; ft and 7 , rich clove brown. Sections approximately 

 parallel to the orthopinacoid (100) show no pleochroism because they contain 

 the ft and 7 axes. 



The amount of hornblende varies in different specimens. In some 

 it is almost as abundant as the augite ; in others it is rare or altogether 

 absent. The hornblende usually occurs on the margin of the augite as if 

 crvstals of the former mineral had been more or less completed by the latter. 

 The boundary of the two minerals is often very irregular ; tongues and 

 flecks of hornblende penetrate the augite and are sometimes found 

 completely isolated in the latter mineral near the zone of contact. 



The two minerals have the vertical axis and plane of symmetry in 

 common. The pleochroism of the augite is often seen to increase from the 

 centre towards the margin ( see Figs. 1 and 2, Plate VII.) and to 

 approximate in character to the hornblende ; nevertheless the final transition 

 is abrupt. In some varieties of the Inchcolm rock detached crystals of 

 basaltic hornblende occur in which the prism faces (110) and clinopinacoids 

 (010) are sharply defined. Whenever the hornblende which is intergrown 

 with the augite shows definite form the faces are those of the former and not 

 those of the latter mineral. 



The relations between augite and hornblende above described are common 

 to many rocks more or less allied to that of Inchcolm, including certain 

 picrites of Nassau,' 1 ' the teschenites of Moravia, 2) Silesia and Portugal (3) 

 and certain "greenstones" of Cornwall, as for example that of St. Minver. (1) 

 Is it due to a paramorphic change of augite analogous to that which takes 

 place when the green fibrous hornblende, known as -iira/itr, is produced ; or is 

 it a case of the definite intergrowth of two mineral species ? The fact that 

 the brown hornblende here referred to shows the definite external form of 

 that mineral at once shows that this is not analogous to " uralitisation." Is 

 it, however, a case of what may be called " amphibolisation ? " This appears 

 to be the view of MACPHERSOX, who suggests that the pleochroism of the 

 augite may be due to the intercalation of ultra-microscopic lamella? of 

 hornblende. The general opinion of German petrographers is that we have 

 here a case of the intergrowth of two definite mineral species, and this view 

 has been greatly strengthened if not absolutely established by RoHRBACH 

 who has separated and analysed the two minerals from a variety of 

 teschenite. He found that from a powder containing the two minerals a 

 separation could be effected by means of a solution of bi-iodide of mercury 

 and barium. The specific gravity of the hornblende was found to lie between 

 3-364 and 3*370 ; that of the augite between 3'37G and 3'421. The following 

 are the analyses of the two minerals : 



(1) ROSKXBUSCII. Mik. Phy., Band I., (1885), Plate XXI., Fig. I. 



(2) ROHRBACH. T.M.M., Neue Folge, Vol. VII., p. 24. 



(3) MACPHEESOX. B.S.G., 1882, p. 289. 



(4) PHILLIPS. Q.J.G.S.. Vol. XXXIV., p. 478, and Plate XXII., Fig. V. 



